Aim: In this study, we investigated whether environmental factors can effectively control the spatial distribution of various benthic algae, and examined the critical ecological impacts of algae on corals across the South China Sea (SCS). Relationships between benthic algae and environmental factors were assessed, and potential ecological impacts of algae on coral communities were evaluated across spatial scales.

Location: A total of 104 sites at 12 coral reefs in four coral reef regions (CRRs) of the SCS (latitude range of 9–22°).

Taxa: Turf algae, macroalgae (including fleshy macroalgae and Halimeda), crustose coralline algae (CCA), coral, and juvenile coral.

Methods: Using diver-based surveys (2015 to 2018), we investigated four CRRs with different reef distance to mainland (RDM) across the SCS. We obtained field data on benthic algal composition and cover, coral cover and diversity, and juvenile coral density. We also measured and collected the environmental factors (including seawater environmental parameters and reef fish diversity).

Results: The cover of turf algae and CCA gradually decreased and increased, respectively, with an increase in RDM, which had a strong relationship with the gradually changing environmental factors. Random forest models suggested that nutrients, reef fish diversity, seawater transparency and temperature were the most important factors for predicting turf algae and CCA cover. Linear regression analyses showed a significant relationship between the turf algae, macroalgae, and CCA covers, and juvenile coral density.

Main conclusions: Our results showed that gradually changing environmental factors were correlated with the spatial distributions of turf algae and CCA. However, the natural biophysical relationships between macroalgae and environmental factors may be disrupted by turf algal overgrowth and nutrient subsidies. Increases in all benthic algal groups significantly impacted coral recruitment, highlighting the critical role of benthic algae in determining recovery trajectory of the Indo-Pacific reefs that are threatened by human activities.

This field dataset was conducted on 12 coral reefs in four coral reef regions (CRRs) in the South China Sea during summer seasons from 2015 to 2018. Photoquadrat method was used to determine the benthic community composition and cover in the surveyed reefs. At each coral reef, 4–12 transects (50 m long) were deployed at a depth of 2–6 m in coastal reefs and Weizhou Island and of 2–15 m in Xisha Islands and Nansha Islands. At each transect (or site), a 50 m fiberglass measuring tape was fixed to the reef substrate, and sampling quadrats of 0.5 m × 0.5 m were randomly set along the tape. SCUBA divers used OLYMPUS TG-4 cameras to take over 20 photographs per quadrat in a vertical line from the reef substrate (about 0.6 m) for each sampling quadrat, and we set up 6 to 24 quadrats on each transect. A total of 1,450 photoquadrats from 104 sites (one transect was done per site) across 12 reefs were used to calculate the benthic biotic and abiotic cover using Coral Point Count with Excel extensions (CPCe version 4.1) software. Briefly, this software was used to place 50 simple-random points on each photoquadrat, and benthos and substrate were identified beneath each point. Benthic communities were mainly defined as hard corals, benthic algae (turf algae, macroalgae, and CCA functional groups), and other benthic groups such as sponges, Milleporina, Helioporacea, Gorgonians, and soft corals. Macroalgae were differentiated as fleshy macroalgae (e.g., Dictyota, Lobophora), Halimeda, and other calcified macroalgae, while this study did not specially focus on the other calcified macroalgae. An abiotic substrate was defined as one consisting of rubble, sand, sediment, bare rock, and dead coral. Hard coral and macroalgae genera were identified, and turf algae and CCA were identified to the level of functional groups.